Biomass pyrolysis is a process typically involving thermal decomposition of biomass e.g. ligno-cellulosic biomass (wood, switch grass, corn stover, etc.) under elevated heating rates, to produce a biomass pyrolysis product typically comprising non-condensable gases (CH4, CO, CO2), water, a liquid product called pyrolysis oil, and char. The char product is typically separated in a cyclone train. Pyrolysis oil is typically a mixture of condensable vapors as well as particulates—finely divided liquid and semi-liquid droplets and solid particles—carried over through the cyclone train. In order to maximize the liquid product yield, heating rates are preferably higher than the rates at which polycondensation of the reactive intermediates occurs. Following a similar logic, cooling rates for the entire pyrolysis effluent are comparably high.
Pyrolysis oil is extremely reactive upon heating. Attempts to distill it under ambient pressure will typically fail because polycondensation and thermal decomposition reactions become significant even under modest heating. The heavier polycondensation products are usually the most difficult to upgrade to liquid transportation fuels, and pose significant challenges to burner design and fouling prevention when the pyrolysis oil is used directly as a fuel.
The biomass pyrolysis product is typically cooled in trains of short residence time, low transfer area heat exchangers so as to prevent fouling while cooling as quickly as possible. Some form of product separation occurs in these condenser trains based on temperature, although liquid-vapor contact is usually too imperfect to achieve full separation equilibrium.
These designs do not provide a method to continuously dilute or remove deposited or reactive products. As these products accumulate, heat transfer worsens and temperature in the deposited layer increases to the point where polycondensation and thermal decomposition become significant. At the same time, because the heaviest product fraction is usually the most reactive at its condensation point, undesirable polycondensation reactions continue, and are in fact promoted, in the concentrated, warm product film formed in the heat exchangers.
What is needed is a process for recovering the biomass pyrolysis product that provides that both provides a rapid quenching rate for biomass pyrolysis products while minimizing undesired polycondensation and thermal degradation.